For a filter press, it is known to form a filter assembly which consists of a plurality of filter plates arranged alongside one another in pairs. Filter chambers are in each case formed between the filter plates. For this purpose, the filter plates have a central filter area and a sealing rim bordering the central filter area. In the filter press, the filter plates can be moved away from and toward one another, as a result of which the filter chambers can be opened and closed.
Each filter chamber usually has an inlet and an outlet. The suspension to be filtered, which is usually designated pulp, flows into the filter chamber via the inlet. The suspension is then filtered by means of a filter cloth, with the solids being held back in the filter chamber by the filter cloth, resulting in the production of what is known as a filter cake. The filtrate freed of the solids is passed out of the filter chamber via the outlet.
In order to reduce the residual moisture in the filter cake and to shorten the filtration times, there are known filter presses in which at least one filter plate of a filter chamber has a diaphragm. Such a filter plate is known as a diaphragm filter plate. In a filter press having diaphragm filter plates, the pressure filtration initially carried out is followed by press filtration, in which the elastic diaphragms of the filter chambers are subjected to a pressure medium, as a result of which they expand and constrict the space in which the filter cake is located. In this way, the filter cake is compressed, and as a result the residual moisture in the cavities of the filter cake is reduced further.
Diaphragm filter plates for filter presses are described for example in EP 0 357 015 B1 and WO 03/095063 A1.
In order to reduce the residual moisture from the filter cake even further, it is known to heat the filter cake. In the case of any further processing of the filter cake, too, it is important to obtain a filter cake having as little residual moisture as possible.
Filter presses in which the material to be filtered is dried have already been known for a long time. DE 590710 describes a filter press in which the material to be filtered is dried by a heating plate. In that document, the starting point is a filter press in which the heating plates are clamped between the frames of the filter press. However, such an arrangement has the disadvantage that reliable sealing is particularly difficult because the heating plates expand during heating. For this reason, DE 590710 proposes forming the heating plates separately from the filter press frame as self-contained heating elements which are installed in a self-supporting manner in the filter frames, as a result of which the residues to be dried are in contact with the heating plate on all sides.
DE 37 13 419 C2 discloses a further method and a further apparatus for drying a filter cake in a filter press. In this case, the filter cake is heated by heating up the rubber diaphragm by subjecting it to heated compressed air. Furthermore, the filter cake itself can be heated by an electric current which flows from electrodes on one diaphragm to electrodes on an opposite diaphragm.
Furthermore, WO 2006/111379 A1 discloses a heatable and coolable filter plate. When this filter plate is used in a filter press, the diaphragm is arranged between the filter chamber and a diaphragm chamber. The diaphragm chamber is bounded on one side by the diaphragm and on the other side by a plastics basic body. This basic body also has grooves, and as a result the thickness of the basic body with respect to the adjacent filter chamber is relatively small. A heated fluid is passed into the diaphragm chamber. In this way, the filter cake in the filter chamber can be heated through the diaphragm. On the other hand, the filter cake in the adjacent filter chamber can also be heated through the basic body, since heat transfer is possible in particular in the regions having a smaller wall thickness in the grooves.
In order to achieve even better heat transfer from a temperature-control element to the filter cake, it has been proposed to form the temperature-control element from metal. WO 2004/024290 A1 describes, for example, a filter press having a metal heating plate, which is screwed onto a plastics frame. In U.S. 2008/0190831 A1, the heating plates are firmly clamped between a basic body and a screwed-on cover plate. WO 2006/089662 A1 describes a temperature-controllable filter plate, in which at least two corrugated elements, which are connected together, are used as heating or cooling body, said elements forming a cavity, through which a temperature-controllable medium can flow. The heating or cooling body is rigidly connected to a frame in a manner spaced apart from the frame via crosspieces or bolts.
Furthermore, DE 200 06 891 U1 discloses a heating element for a filter press which comprises a plurality of diaphragm filter elements and heating elements which are arranged in an alternating manner in rows and are clamped in a press frame. The heating element comprises a basic body which is closed in a sealed manner on both sides by a respective metal plate, such that branching heating-medium channels are produced.
In the case of filter plates in which a metal heating element is rigidly connected to a plastics frame, the problem arises that when the filter plates are used in a filter press, leaks frequently occur or stresses in and bending of the system composed of the heating element and the frame occur. The reason for this resides in the extremely different coefficients of thermal expansion of metal and plastics material. When the heating element is heated, it expands much less than the plastics frame. In the long run, this causes relative displacements between the heating element and the frame, which lead to the sealing elements being damaged.
In order to solve this problem, EP 0 676 225 A1 proposes forming the filter press entirely from metal, with the plates then being connected peripherally to an associated frame by weld seams. However, such filter plates are very expensive.
Further, EP 1 654 048 B1 proposed fastening two metal heating plates to a basic body via a central hole and a flange. By way of this type of fastening, it is possible for the heating plate to thermally expand largely independently of the basic body without causing tension between the basic body and the heating plate.